This invention relates to a dispensing system for liquids, especially polyurethane foam precursor chemicals, and more particularly is directed to such a system wherein the chemicals are heated under precisely controlled conditions in the conducting lines thereof.
In the work which led to the present invention, I have been dealing principally with polyurethane foam components which require heating above ambient temperatures in order to result in a useful foam product. While such chemicals are well-known to those skilled in this art, in order to fully explain the operation of this system I would briefly allude to these reactants.
Polyurethane foams are normally produced by the reaction of two principal constituents generically classified as polyols and polyisocyanates.
The polyols generally are selected from the group consisting of polyethers, polyesters, polythioethers, polyesteramides, alkylene glycols and polyisocyanate modifications thereof, which materials are characterized by a molecular weight greater than 500 and which have at least two reactive hydroxyl groups per molecule. Examples of the foregoing are poly (oxyethylene) glycols and poly (oxypropylene) glycols or copolymers of these materials which collectively may be referred to as poly (oxyalkylene) ethers. As is well-known to the chemists skilled in this art, there are numerous compounds which fall within this "polyol" category and the literature thereon is quite extensive.
The other principal reactant in the polyurethane foam making reaction are aromatic or aliphatic polyisocyanates. Also included are substitute of aromatic polyisocyanates. Representative examples of these materials are 2, 4-toluene diisocyanate; 2, 6-toluene diisocyanate; ethylene diisocyanate; ethylidine diisocyanate; propylene-1, 2-diisocyanate; butylene-1, 2-diisocyanate; hexylene-1, 6-diisocyanate; cyclohexylene-1, 2-diisocyanate and many others.
In addition to the polyols and polyisocyanates, the usual polyurethane precursor materials also contain one or more of blowing agents, surfactants, catalysts, fillers and stabilizers. These are usually contained in either the polyol or polyisocyanate constituent.
Certain polyurethane foam compositions are usable at ambient temperatures but these are not basically the materials which are to be used in the present dispensing system. I am principally interested in using those foam materials herewith wherein a modest, but very carefully controlled heating thereof is required. To understand why such careful heating is necessary requires some additional discussion.
While polyurethane foam has become a staple item of commerce it should be recognized that the production of such foam involves the use of rather sensitive chemicals, somewhat difficult to work with, and that it is extremely important, if acceptable foams are to result, that there be good mixing of well measured, fairly exacting amounts, of the two basic materials. If the two constituents are not well mixed prior to dispensing or prior to the initiation of the foaming reaction, one does not obtain an acceptable foam. Furthermore, if there is an excess of one reactant over the other acceptable foams will not result.
These general statements are especially true in the area of polyurethane foaming in place packaging where most of my efforts in the present invention have been directed. The foam going into the package has to be correct or the package has to be remade or may fail in use which are not particularly attractive from either a functional or economic standpoint.
While the polyurethane foaming reaction is usually exothermic in nature most of the regularly used chemical components have to be moderately heated prior to the initiation of the reaction. This is normally done, of necessity for several reasons. The constituents are usually viscous and heating brings one or both of them down to a viscosity range where it or they can be reasonably handled. Also, since the two components normally widely differ in viscosity, but change in viscosity differently as a function of increasing temperature by controlling the temperature of the reactants, one can substantially match the two viscosities. This is important in obtaining good mixing of the two chemicals.
And, of course, heating is important in "triggering" the foam reaction.
It is, of course, understood that the heating cannot be too high without serious detrimental effects. As a practical matter with the polyurethane reactions that I have been using, a temperature of 130.degree.F is preferred.
In the past the materials have been heated to the appropriate temperature in their containers. This requires heaters for the containers, heavily insulated lines, and still does not assure that the temperature is correct at the critical point of mixing and pour. Also, the heating of the containers detracts from the instantaneous operation of the system -- when a new container replaces a depleted one, the operator must wait until the appropriate temperature is achieved. As the containers have to be preheated; or for example one might have to leave the heat on overnight in order to assure that the unit will function properly at the start of operations the next morning. Combined with this are the limitations on hose length between container and pour point. Cooling down at the latter has to be avoided.
All of the foregoing problems are overcome by the use of the present invention. The reactants are electrically heated in the conduits thereby making it unnecessary to heat or preheat in the container and reactant temperatures are carefully sensed at and appropriately fed back from, the dispensing end of the system.
In view of this a principal object of my invention is to provide a polyurethane foam dispensing system wherein the chemical reactants thereof are brought to a carefully controlled optimum reaction temperature by electrical heating within the conduits thereof.